Fixed bed reactors are commonly used in the refining and chemical industries for chemical conversion operations. These reactors are vessels with catalysts packed therein. The feed typically flows down through the reactor where it comes into contact with the catalyst and undergoes the desired conversion reactions. When these involve reaction with hydrogen (H2), the term hydroprocessing or hydroconversion is used.
Hydroprocessing reactions include addition of hydrogen (H2) to a feed molecule to achieve reduction chemistry (hydrogenation) and can also involve cleavage of the molecule (“destructive hydrogenation” or hydrogenolysis). Hydrogenation reactions include saturation of carbon-carbon double bonds (e.g. conversion of olefins to paraffins; benzene rings to cyclohexane rings) and functional group transformations (e.g. conversion of aldehydes to alcohols; nitro compounds to amines).
Examples of hydrogenolysis reactions include conversion of organo-sulfur compounds such as mercaptans to paraffins and hydrogen sulfide, wherein the sulfur atom is cleaved off the feed molecules. Other examples include conversion of organo-nitrogen compounds and organo-oxygen compounds (also referred to as oxygenates) to hydrocarbons, wherein the nitrogen and oxygen atoms in the feed molecules are cleaved as ammonia and water/carbon oxides, respectively. In the refining industry, this class of hydroprocessing reactions is referred to as hydrotreating. Hydrotreating reactions are typically used to remove heteroatoms such as sulfur, nitrogen, and oxygen from hydrocarbons. Depending on the heteroatom removed, the hydrotreating operation may be referred to as hydrodesulfurization (HDS), hydrodeoxygenation (HDO), or hydrodentirogenation (HDN).
Other hydroprocessing reactions include hydrocracking, wherein a large feed molecule breaks into smaller molecules, and hydroisomerization, wherein a straight-chain molecule is converted to a branched molecule of substantially the same average molecular weight and a similar boiling point range. More than one hydroprocessing reaction may take place at the same time in the presence of a single catalyst. When the goal is improving the quality of a fuel stock (e.g. improvement of emission characteristics, low temperature flow properties, or thermo-oxidative stability), hydroprocessing operations are also referred to as “upgrading.”
Although most hydroprocessing deals with upgrading of petroleum fractions, its use in biofuel production processes has attracted much interest over the past several years.
During hydroprocessing of both petroleum fractions and bio-oils, the pressure drop across the fixed bed reactor rises with time on stream. Without being bound by theory, this is believed to be due to deposition of solids present in the feed as well as the deposition of solids formed during thermal and/or chemical conversion of the liquid feed. Examples of the former include suspended solids that were not filtered upstream of the reactor or solids that drop out of solution, while the latter includes coke and polymerization products. In either case, as solids fill the reactor void (fouling the reactor, hence the term “foulant”) the pressure drop rises. The foulant produced by petroleum hydroprocessing cannot be removed through use of an organic solvent or other dispersing agent. Once the maximum safe operating pressure drop is reached, the reactor needs to be shut down and the deposits skimmed (physically removed from the reactor when accumulating at the top) or the catalyst replaced (even if the catalyst is still active).
There are various options for mitigating fouling and pressure drop in fixed bed hydroprocessing reactors. One such option includes grading the bed with several layers of catalyst/inert media, with the largest on top and smallest in the bottom. Instead of concentration of the solids in the top section of the reactor resulting in a more rapid pressure drop increase, by grading the bed the solids tend to spread out within the graded section of the reactor.
Another option involves using a fixed bed reactor media in the form of wagon wheels, Raschig rings, and other shapes having high void fractions (“high void” means typically 50% or higher void fraction). Reactors containing top layers of these media can hold more solids before reaching the pressure drop limit. These high void media, including high void/low activity catalysts (e.g. “active rings”), may be graded to provide systems achieving significantly longer pressure drop limited run lengths. “Low activity catalysts” are those catalysts with less than 5 wt % of the active catalytic metal. Some of the top bed grading media contains internal macro-porosity capable of capturing very fine particulates that can nonetheless agglomerate within the fixed bed reactor void space and cause pressure drop issues.
An internal bypass apparatus for fixed bed reactors is another option. The bypass can include pipes which allow a hydrotreater feed to bypass the fouled section of the reactor so that nothing runs through the fouled section. The pipes can be equipped with rupture disks that burst open when the pressure drop reaches a value just below the operating limit, thus extending run length.
Although the above pressure drop mitigation methods and equipment can be useful for hydroprocessing reactor feeds comprising bio-oils, additional methods more suitable for solid deposits characteristic of bio-oils are desirable.